Musnad Ahmad 1156:100

Talq ibn Ghannam ibn Talq narrated to us, Sa'id ibn 'Uthman al-Warraq narrated to us, from Abu Salih, who said: I entered upon Umm Salamah, and a son of her brother entered upon her, and he prayed two rak'ahs in her house. When he prostrated, he blew the dust away. Umm Salamah said to him: "O son of my brother, do not blow, for I heard the Messenger of Allah, peace and blessings of Allah be upon him, say to a servant of his who was called Yasar, and he was blowing: 'Make your face dusty for Allah.'"